This Recommendation covers the technical requirements for multimedia communications systems in those situations where the underlying transport is a Packet-Based Network (PBN) which may not provide a guaranteed Quality of Service (QoS). These packet-based networks may include Local Area Networks@ Enterprise Area Networks@ Metropolitan Area Networks@ Intra-Networks and Inter-Networks (including the Internet). They also include dial up connections or point-to-point connections over the GSTN or ISDN which use an underlying packet-based transport such as PPP. These networks may consist of a single network segment@ or they may have complex topologies which incorporate many network segments interconnected by other communications links. This Recommendation describes the components of an H.323 system. This includes Terminals@ Gateways@ Gatekeepers@ Multipoint Controllers@ Multipoint Processors and Multipoint Control Units. Control messages and procedures within this Recommendation define how these components communicate. Detailed descriptions of these components are contained in clause 6. H.323 terminals provide audio and optionally video and data communications capability in point-to-point or multipoint conferences. Interworking with other H-series terminals@ GSTN or ISDN voice terminals@ or GSTN or ISDN data terminals is accomplished using Gateways. See Figure 1. Gatekeepers provide admission control and address translation services. Multipoint Controllers@ Multipoint Processors and Multipoint Control Units provide support for multipoint conferences. The scope of H.323 does not include the network interface@ the physical network or the transport protocol used on the network. Examples of these networks include but are not limited to: ?C Ethernet (IEEE 802.3); ?C Fast Ethernet (IEEE 802.3u); ?C FDDI; ?C Token Ring (IEEE 802.5); ?C ATM.